Hitherto, as packaging materials to be sterilized by hydrothermal treatment, there have been frequently used laminates prepared by bonding an aluminum foil and a film with each other, and deposited films prepared by vapor-depositing aluminum on a surface of the film. However, since such packaging materials are opaque due to aluminum used therein, there is caused such a problem that resultant packages filled with contents are no longer subjected to inspections for metal foreign materials in the contents and appearance of the contents.
Consequently, in recent years, there have been proposed many gas-barrier films having a high transparency which are prepared by forming an inorganic thin film made of silicon oxide, aluminum oxide or magnesium oxide which has a thickness of 10 to 100 nm, on the surface of a base plastic film. These gas-barrier films are usually in the form of a laminate further including a separate plastic film layer laminated on the inorganic thin film, and are used as packaging materials for foods, medicines or the like.
It has been expected that the above gas-barrier laminates are used as retort packaging materials. However, in such application fields, the packaging materials have been required to still maintain the gas-barrier property even after being subjected to hydrothermal treatment. However, the inorganic thin film formed in the above laminates tends to be readily broken upon the hydrothermal treatment, resulting in remarkable deterioration in gas-barrier property thereof. It is considered that the problem is caused by change in volume of the base plastic film due to shrinkage thereof when subjected to hydrothermal treatment, or oxidation of the inorganic thin film. The gas-barrier property of the laminates may be maintained by increasing the thickness of the inorganic thin film, for example, up to about 100 nm or more. In this case, however, the productivity of the laminates tends to be deteriorated, resulting in increased production costs. Further, there may arise problems such as poor transparency of the obtained laminates.
To solve the above problems, there have been proposed the method of subjecting a deposited film including an inorganic thin film to heat treatment as a post treatment thereof for a short period of time, e.g., for a period of from about several seconds to about one minute in order to enhance an adhesion between the inorganic thin film and the other plastic film layer (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 55-84332(1980)), and the method of forcibly irradiating a deposited film with a visible light for a long period of time to enhance a transparency of the film and stabilize a gas-barrier property thereof (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 8-197674(1996)). However, these conventional methods have failed to attain the effect of improving a gas-barrier property of the gas-barrier films having the inorganic thin film after subjecting these films to the hydrothermal treatment.
In addition, there have been proposed the method of subjecting a deposited film to water or moisture absorption treatment and then to heat treatment in order to maintain a transparency of the film and improve a gas-barrier property thereof (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 2-299826(1990)), and the method of heat-treating a laminated film produced by laminating the other film on an inorganic thin film formed on a biaxially stretched film through an adhesive resin layer for a short period of time at a temperature of from a second-order transition point to a melting point of the biaxially stretched film, more specifically at about 200° C. at which the adhesive resin layer can be melted, while preventing heat shrinkage of the film, in order to improve a gas-barrier property of the laminated film (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 8-300549(1996)). Although the films obtained in these conventional methods are improved in gas-barrier property, when a laminate using the deposited films is used as a retort packaging material, a gas-barrier property thereof after subjected to the hydrothermal treatment tends to be still insufficient.
Further, it is known that a deposited polyester film having a polyester-based anchor coat layer which undergoes heat history at not lower than the glass transition temperature during the production process of the deposited polyester film, can exhibit a good adhesion between the based polyester film and the deposited layer even when the deposited polyester film is subjected to hydrothermal treatment (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 3-16728(1991)). However, the heat history imparted to the film in the above method is only a general heat treatment conducted upon ordinary vapor deposition steps or film-stretching/heat-treating steps during the production process of deposited films.